Nakajima Racing drivers Tadasuke Makino and Alex Palou qualified on the front row and quickly sprinted away from the rest of the field, both having chosen to start on soft tyres.
However, the first safety car - deployed on lap 9 of 43 after Impul driver Ryo Hirakawa and Real Racing’s Tristan Charpentier crashed at 130R - changed the complexion of the race.
Cassidy, who had pitted a few laps prior to the safety car for the faster soft tyre, suddenly found himself behind effective race leader Makino, who was left on the slower medium tyre after his own stop - and with his advantage wiped out.
Palou had served a drive-through penalty for not following the correct start procedure shortly before the SC and was effectively ruled out of contention.
Cassidy made quick work of Makino after the SC pulled in, with Dandelion's Naoki Yamamoto also passing him a few corners later.
Two more safety cars were deployed in the next 10 laps - the first for separate offs for Kazuki Nakajima and Harrison Newey, and the other for Palou losing drive - before racing resumed.
It was Kamui Kobayashi, yet to stop, who led the field at this point, with Cassidy and Yamamoto quickly passing the other non-stoppers to move up to second and third on the road.
Kobayashi had started on the soft tyre and was aiming to extend his advantage enough to still come out in front, but his hopes were dashed when the safety car made a fourth appearance on lap 27 - this time for Makino suffering a spectacular right-rear wheel failure on the exit of Spoon curve.
Ex-Formula 1 driver Kobayashi stayed out when the SC was deployed and then set about rebuilding a gap to the rest, extending his advantage to 11 seconds by the penultimate lap.
However, that wasn’t enough and he returned to the track down in 10th place when he finally pitted.
With Kobayashi dropping down the order, Cassidy assumed the race lead for the first time and took the chequered flag by 1.749s, claiming his second victory in the series.
Last year’s champion and Dandelion driver Yamamoto finished second after successfully holding off a late charge from Kondo’s Kenta Yamashita.
Mugen’s Tomoki Nojiri was disadvantaged by being on the slower medium tyre for the second half of the race, but held off advances of his Team LeMans rival Kazuya Oshima to take fourth.
Reigning Japanese F3 champion Sho Tsuboi finished sixth on his series debut, ahead of Yuji Kunimoto (Kondo), who was able to charge his way through the field after pitting on lap 31.
B-Max by Motopark driver Lucas Auer collected the final championship point in eighth after coming on top in a private battle with fellow Red Bull junior Dan Ticktum.
Mugen’s Ticktum was one of the star performers in the opening laps of the race, having scythed his way inside the top 10 from 16th on the grid.
However, he lost a chunk of time during his pitstop under the safety car and then dropped to the rear of the pack after coming in again for fresh tyres during the next SC period.
Formula 2 race winner Artem Markelov also endured a torrid Super Formula debut, the Russian driver finishing 11th after having to serve a drive-through penalty for overtaking under the safety car.
He had started the race from 20th and last after crashing out in qualifying.
Only 12 drivers finished what was a race of attrition, as several cars retired either due to technical failures or crashes at the start of series’ new SF19 era.
: Oshima, who had originally crossed the finishing line in fifth, has had 30 seconds added to his total time, dropping him all the way down to 12th.
Oshima's penalty promotes Tsuboi up to fifth, Kunimoto to sixth and Auer to seventh. Ticktum picks up the final championship point in eighth.
All Super Formula races are streamed live and for free on Motorsport.tv.
|1||Nick Cassidy||TOM'S||43 laps|
|2||Naoki Yamamoto||Dandelion Racing||1.79|
|3||Kenta Yamashita||Kondo Racing||2.399|
|6||Yuji Kunimoto||Kondo Racing||10.825|
|7||Lucas Auer||B-Max Racing with Motopark||13.989|
|10||Artem Markelov||Team LeMans||37.082|
|11||Nirei Fukuzumi||Dandelion Racing||37.977|
|12||Kazuya Oshima||Team LeMans||39.057|
|DNF||Tadasuke Makino||Nakajima Racing|
|DNF||Alex Palou||Nakajima Racing|
|DNF||Harrison Newey||B-Max Racing with Motopark|
|DNF||Yuhi Sekiguchi||Team Impul|
|DNF||Ryo Hirakawa||Team Impul|
|DNF||Tristan Charpentier||Real Racing|